Sheet metal forming processes are known in the art and typically include forcing a sheet metal workpiece against a forming tool surface, sometimes called a die surface. In electromagnetic forming (EMF) of sheet metal the workpiece is rapidly propelled by a momentary electromagnetic force over a short distance against the forming surface at velocities far in excess of those found in a conventional stamping technique. Typically, the movement and deformation of the workpiece is completed within a few tens of microseconds. EMF is usually applied to sheet metal workpieces that have typical sheet or foil thicknesses up to about 3 millimeters thick and frequently to workpieces less than one-half millimeter in thickness.
In a practice of EMF, a low electrical resistivity (e.g., less than about 0.15 micro-ohm meter) sheet metal workpiece is positioned close to or against a forming tool surface. Such materials include, for example, sheets of copper, aluminum, and some of their alloys. For example, an inductive coil electromagnetic actuator is used. It is positioned close to the opposite side of the highly conductive sheet metal. A strong electrical current is discharged through the windings of the coil to generate, momentarily, a strong electromagnetic field. That field induces an opposing electrical current in the workpiece. The opposing magnetic fields between the stationary coil and the workpiece sheet accelerate the workpiece to a high velocity and upon impact it stretches the sheet into conformance with the tool surface. As an example, U.S. Pat. No. 7,076,981 describes a use of electromagnetic forming in shaping networks of serpentine flow passages in thin metal flow field plates for a hydrogen/oxygen fuel cell.
In some instances, the desired sheet metal workpiece may lack suitable electrical conductivity to respond to the magnetic field and be driven against the forming surface by the discharge of the electromagnetic actuator. In this situation, a low resistivity driver plate may be placed between the electromagnetic actuator and the sheet metal. The driver plate reacts to the electromagnetic field and drives the sheet metal against the forming surface. Both the driver plate and the metal workpiece are permanently deformed in the process. So the driver plate must be separated from the formed product and either discarded or recycled, and the shaped sheet metal product is advanced to the next stage in its manufacturing process.
Electromagnetic forming can achieve strain rates of the order of 105 sec−1 and sheet velocities in the range of 50 to 300 m/s. Such strain rates in sheet metal workpieces may improve the formability of the workpiece material. The high strain rates may increase the ability to make sharp and deep features in the workpiece while decreasing spring-back of the formed sheet and wrinkling of its features. Thus, there is a need for a means of conducting electromagnetic forming of sheet metal materials of higher electrical resistivity without having to use and discard (or restore to their original flat condition) low resistivity driver plates after each forming operation.